1. Field of the Invention
The present invention relates to a free cutting steel containing controlled inclusions. Mechanical anisotropy of the steel is decreased by controlling the form of the inclusions without impairing good machinability thereof.
This invention is applicable to various steels such as carbon steel and alloyed steel for structural use, stainless steel, heat-resistant steel, bearing steel, tool steel and spring steel. Application to the stainless steel and the heat-resistance steel gives free cutting steels having good formability in cold forging.
This invention also relates to the method of making the above free cutting steel and products produced by hot working the free cutting steel.
2. State of the Art
In order to achieve good machinability of steel, it is practiced to add a machinability-improving element such as a metal, e.g., Pb, Bi, Ca and Te, or S or Se to steel composition. In the S- or Se-free cutting steel, which are the most widely used, these elements form inclusions of the composition MnS or MnSe, or, if both of them are used, Mn(S, Se).
Though the melting points of MnS and MnSe are so high as above 1300.degree. C., the inclusions maintain plasticity even at a lower temperature above 900.degree. C. If a free cutting steel containing such inclusions are hot worked, particles of the inclusion are elongated and, as a result, there arises a trouble that anisotropy in mechanical properties of the steel such as tensile strength increases.
To lighten this difficulty, addition of Ti, Zr or REM(rare earth metals) has been made to decrease plasticity of the inclusions so that the elongation of the particles during the hot working may be lowered. This resulted in an increase in the hardness of the inclusion, however, it inevitably weakens the expected effect of improving machinability of the steel.